1. Field of the Invention
The invention relates in general to the field of hot forming articles and especially to a method of and an apparatus for hot forming of at least a part of an article, and to articles which at least in part are hot formed.
2. Description of Related Art
In the field of hot forming of articles exists a huge experience, especially in the field of drawing of fiber optic articles, as single fibers, hollow fibers, multi fiber rods or of optical face plates, inverters, and tapers.
Optical fibers or multi fiber rods may consist of glass or synthetic material as plastic, especially polymeric material, or any combination of these materials.
Further, hot ductile material as glass ceramic material is heated to initiate micro crystallization or ceramization processes.
In shaping processes, especially hot forming or hot post-processing, semi-transparent or transparent glasses and/or glass ceramic materials and plastics are heated up to a processing point where a viscosity η between 1014.5 and 104 dPas is encountered or beyond that. Semi-transparent or transparent glasses and/or glass ceramics, for the setting-in of certain material properties, for example ceramization, are heated mostly to temperatures which lie preferably above the lower cooling point at a viscosity η of about 1014.5 dPas.
Typical, lower cooling points for glasses can be, depending on the type of glass, between 555 K and 1063 K, and typically the processing point can be up to 1978 K. For plastics the cooling and the processing point can be even much lower, typically being in the range of 250 K up to 580 K.
Hitherto according to the state of the art semi-transparent or transparent glasses and for glass ceramics, for example for ceramization, were heated preferably with surface heating by hot air and/or long-wave infrared is radiation. As surface heating there are designated processes in which at least 50% of the total heat output of the heat source is introduced into the surface or surface-near layers of the object to be heated.
Since most glasses in this wavelength range exhibit an absorption edge, 50% or more of the radiation output is absorbed by the surface or in surface-near layers.
As glass or glass ceramic material has as a rule a very low heat conductivity in the range 1 W/(mK), it was believed that glass or glass ceramic material with increasing thickness must be heated up more and more slowly in order to keep tensions in the glass or glass ceramics low. It was believed that when a homogeneous heating-up of the glass or of the glass ceramic is not achieved or is only inadequately successful, then this unfailingly would result in inhomogeneities in the process and/or in the product quality and/or in destroying the material.
From DE 42 02 944 C2 there has become known a process and a device comprising IR radiators for the rapid heating of materials which have a high absorption above 2500 nm. In order to rapidly introduce the heat given off from the IR radiators, into the material, DE 42 02 944 C2 proposes the use of a radiation converter from which secondary radiation is emitted with a wavelength range shifted into the long-wave direction with respect to the primary radiation.
A heating of transparent glass homogeneous in depth with use of short-wave IR radiators is described in U.S. Pat. No. 3,620,706. The process according to U.S. Pat. No. 3,620,706 is based on the principle that the absorption length of the radiation used in glass is very much greater than the dimensions of the glass object to be heated, so that the major part of the impinging radiation is transmitted through the glass and the absorbed energy per volume is nearly equal at every point of the glass body. What is disadvantageous in this process, however, is that no homogeneous irradiation over the surface of the glass objects is ensured, so that the intensity distribution of the IR radiation source is replicated on the glass to be heated. Moreover, in this process only a small part of the electric energy used is utilized for the heating of the glass.
In production processes of fiber optical image or light guides made of glass or plastic a preform which consists of multi-component transparent, semi-transparent and/or opaque glasses or plastics is heated by means of an electrical resistance heating and drawn to a fiber/multi fiber rod. If necessary, the developing fibers/fiber rods can be brought together again afterwards to build new preforms, which are again drawn to fibers/fiber rods. These new fibers/fiber rods thereby contain a multitude of the fibers/fiber rods drawn in the previous step. After several of such process steps one can get a fiber rod with several million single fibers, which can be used as image guides.
It proved to be difficult even for preforms with diameters greater than 50 mm to achieve a temperature distribution which is as homogenous as possible within the preform consisting of multi-component transparent, semi-transparent and/or opaque glasses or plastics, in a way that the drawing to a fiber/multi fiber rod does not lead to irregularities within the fibers/fiber rods. With plastics especially, the thermal damage at the surface has to be mentioned. Furthermore, the heating speed of the preform is crucial, since for drawing the fiber the preform can only be inputted and drawn as fast as mass is sufficiently heated.
A conventional resistance heating with temperatures of typically 1300 K at the heating coil is inadequate in this respect especially with preforms of diameters larger than 50 mm, since the emitted radiation is in a wavelength range absorbed on the surface or in layers near the surface of the glass or the plastic (penetration depth<1 mm), the heating thereby being a surface heating. The inner part of the preform has to be heated completely by thermal conduction. Since glass and plastic have a poor thermal conductivity, a lot of time is needed for the heating process especially of preforms with large dimensions, because it is necessary also for the inner part of the preform to reach the temperature needed for drawing and to reduce the average temperature within the preform. Also the subsequent drawing of the fiber or rod therefore can be done only with a limited speed, since new material of the preform has to be provided and heated continuously.
A possibility to avoid these difficulties should be, according to EP 1 171 392, to replace the heating elements usually used until now by appropriate short wave IR radiation sources arranged in a radiation cavity, the color temperature of which is above 1500 K and thus, a maximum radiation intensity at a wavelength shorter than 2000 nm. In this range, many glasses and plastics are almost transparent and absorb little of the incoming radiation. That way, a depth effective heating is achieved. Only negligible temperature inhomogenieties still arise, because every volume element absorbs the almost same amount of radiation. These amounts are each very small, since the absorption is very low.
By means of multiple reflections of the short-wave IR radiation at the walls of the radiation cavity the part of radiation reaching the material to be heated indirectly is above 50%. Furthermore, the associated rise in efficiency results in extremely high heating rates without damage to the material or disturbing temperature gradients.
However, in the production of image guides many, up to several million, single fibers/fiber rods are brought together to build a preform and often additionally single fibers of high-absorbing material are placed in between for contrast increase. The penetration depth from the side for short-wave IR radiation is drastically reduced by these high-absorbing fibers/fiber rods and by the Fresnel-reflection at the transparent fibers. That is because the radiation hitting from the side during the heating has to cross each single fiber/multi fiber rod, thereby passing two surfaces. Since there is a change in the refractive index at each surface crossing from glass or plastic to air or vice versa, according to Fresnel about 4% of the incoming radiation is reflected at each surface, which amounts to a total of 8% of the radiation per single fiber.
The reflected radiation hits surfaces of fibers crossed on the way to the inner region of the fiber again on the way back towards the periphery and is thereby again partly reflected, so that the effective penetration depth of the radiation into the inner region of the preform is up to 30 mm depending on the design. That means that in contradiction to the teachings of EP 1 171 392 neither is the preform completely penetrated by all of the IR radiation, so that an important amount of the radiation is reflected at the opposite wall of the radiation cavity, nor is only a small amount of the incoming radiation absorbed by the preform. Rather, an important amount of the total radiation is absorbed by the preform through multiple reflections and absorptions within the preform. A homogenous heating according to EP 1 171 392 especially of large preforms therefore is not possible.
It is an object of the invention, to improve the performance of hot forming processes.